Native VP9 decoder is now in the Git master branch

VP9 is the next generation video codec developed by Google. It is the core part of WebM, and is designed to replace VP8 and compete with HEVC/H.265. It doubles the quality of VP8 at any given bit rate. Previously, FFmpeg only supports decoding VP9 though the reference decoder libvpx, but the new native decoder brings more stability and performance gains when compared with libvpx.